Cellectis agreed with the FDA to the following main revisions to be
implemented in Phase 1 UCART123 protocols to lift the hold:

Decrease of the cohort dose level to 6.25x104 UCART123
cells/kg;

Decrease of the cyclophosphamide dose of the lympho-depleting regimen
to 750 mg/m²/day over three days with a maximum daily dose of 1.33
grams of cyclophosphamide;

Inclusion of specific criteria at Day 0, the day of UCART123 infusion,
such as no new uncontrolled infection after receipt of
lymphodepletion, afebrile, off all but replacement dose of
corticosteroids, no organ dysfunction since eligibility screening;

Provision to ensure that the next three patients to be treated in each
protocol will be under the age of 65;

Provision to ensure that the enrollment will be staggered across the
UCART123 protocols AML123 and ABC123: at least 28 days should elapse
between the enrollments of two patients across the two studies.

Cellectis is currently working with the investigators and clinical sites
to obtain IRB’s approval on the revised protocols and resume patient
enrollment.

About UCART123

Our first wholly-controlled product candidate, UCART123, is a gene
edited T-cell investigational drug that targets CD123, an antigen
expressed at the surface of leukemic cells in AML, as well as on
leukemic and other tumoral cells in BPDCN. Cellectis received in
February 2017 an Investigational New Drug (IND) approval from the U.S.
Food and Drug Administration (FDA) to conduct Phase 1 clinical trials
with UCART123 in patients with acute myeloid leukemia (AML) and blastic
plasmacytoid dendritic cell neoplasm (BPDCN). This marks the first
allogeneic, “off-the-shelf” gene-edited CAR T-cell product candidate
that the FDA has approved for clinical trials.

AML is a devastating clonal hematopoietic stem cell neoplasm that is
characterized by uncontrolled proliferation and accumulation of leukemic
blasts in bone marrow, peripheral blood and, occasionally, in other
tissues. These cells disrupt normal hematopoiesis and rapidly cause bone
marrow failure and death. In the U.S. alone, there are an estimated
19,950 new AML cases per year, with 10,430 estimated deaths per year.

The clinical research at Weill Cornell is led by principal investigator
Dr. Gail J. Roboz, Professor of Medicine at Weill Cornell Medicine and
Director of the Clinical and Translational Leukemia Programs at Weill
Cornell Medicine and NewYork-Presbyterian.

BPDCN is a very rare and aggressive hematological malignancy that is
derived from plasmacytoid dendritic cell precursors. BPDCN is a disease
of bone marrow and blood cells but also often affects skin and lymph
nodes.

Cellectis is a clinical-stage biopharmaceutical company focused on
developing a new generation of cancer immunotherapies based on
gene-edited T-cells (UCART). By capitalizing on its 17 years of
expertise in gene editing – built on its flagship TALEN® technology and
pioneering electroporation system PulseAgile – Cellectis uses the power
of the immune system to target and eradicate cancer cells.

Using its life-science-focused, pioneering genome engineering
technologies, Cellectis’ goal is to create innovative products in
multiple fields and with various target markets. Cellectis is listed on
the Nasdaq market (ticker: CLLS) and on the NYSE Alternext market
(ticker: ALCLS). To find out more about us, visit our website: www.cellectis.com

Talking about gene editing? We do it. TALEN® is a registered trademark
owned by the Cellectis Group.

Disclaimer

This press release contains “forward-looking” statements that are based
on our management’s current expectations and assumptions and on
information currently available to management. Forward-looking
statements involve known and unknown risks, uncertainties and other
factors that may cause our actual results, performance or achievements
to be materially different from any future results, performance or
achievements expressed or implied by the forward-looking statements.
Further information on the risks factors that may affect company
business and financial performance, is included in filings Cellectis
makes with the Security Exchange Commission from time to time and its
financial reports. Except as required by law, we assume no obligation to
update these forward-looking statements publicly, or to update the
reasons actual results could differ materially from those anticipated in
the forward-looking statements, even if new information becomes
available in the future.

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